Hydraulic valve lash adjuster for a valve train of an internal combustion engine

ABSTRACT

A hydraulic valve lash adjuster for a valve train of an internal combustion engine, for example for an automotive vehicle, said hydraulic valve lash adjuster comprising a piston ( 2 ) that is guided for displacement in a piston housing ( 4 ) and elastically supported against said housing, said piston ( 2 ) comprising a low pressure chamber ( 3 ) that communicates via an axial opening ( 26 ) in a piston bottom ( 19 ) with a high pressure chamber ( 5 ) defined by the piston housing ( 4 ) and the piston ( 2 ), and further comprising a control valve ( 6 ) acting between said pressure chambers ( 3, 5 ), said control valve ( 6 ) comprising a valve closing body ( 7 ) that can be brought to bear sealingly against a valve seat ( 24 ) that surrounds the axial opening ( 26 ) on a piston body undersurface ( 28 ) and is received in a closing body cap ( 9 ) whose cap bottom ( 21 ) acts as a stroke limitation of the valve closing body ( 7 ), and said control valve ( 6 ) further comprising a control valve spring ( 8 ) that loads the valve closing body ( 7 ) in opening direction, and, during a collapsing movement between the piston ( 2 ) and the piston housing ( 4 ), said valve closing body ( 7 ) being hydraulically loadable in closing direction against the biasing force of the control valve spring ( 8 ) by a pressure build-up in the high pressure chamber ( 5 ). 
     To improve the valve lash adjuster so that undesired operational fluctuations are reduced, the valve closing body ( 7 ) is configured as a needle-shaped, elongate sealing piston whose valve seat-proximate end comprises a plate-shaped sealing surface ( 10 ) that cooperates with a correspondingly configured sealing surface ( 25 ) of the valve seat ( 24 ) and whose valve seat-distal end comprises a surface of application ( 12 ) for hydraulic loading, and the valve closing body ( 7 ) substantially fills a space volume of the closing body cap ( 9 ) surrounding the valve closing body ( 7 ).

FIELD OF THE INVENTION

The invention concerns a hydraulic valve lash adjuster for a valve train of an internal combustion engine, for example, for an automotive vehicle, said hydraulic valve lash adjuster comprising a piston that is guided for displacement in a piston housing and elastically supported against said housing, said piston comprising a low pressure chamber that communicates via an axial opening in a piston bottom with a high pressure chamber defined by the piston housing and the piston, and further comprising a control valve acting between said pressure chambers, said control valve comprising a valve closing body that can be brought to bear sealingly against a valve seat that surrounds the axial opening on a piston body undersurface and is received in a closing body cap whose cap bottom acts as a stroke limitation of the valve closing body, and said control valve further comprising a control valve spring that loads the valve closing body in opening direction, and, during a collapsing movement between the piston and the piston housing, said valve closing body being hydraulically loadable in closing direction against the biasing force of the control valve spring by a pressure build-up in the high pressure chamber.

BACKGROUND OF THE INVENTION

Hydraulic valve lash adjusters are used in valve trains of internal combustion engines in automotive vehicles to automatically adjust a valve lash that arises from thermal expansion, manufacturing tolerances and wear of the transmitting elements during a loading of a gas exchange valve by a cam. For this purpose, in the case of common lash adjusters, the respective mechanical transmitting element that transmits a cam lift of the cam to the gas exchange valve comprises a piston that is guided with sealing lash for displacement in a housing and is elastically supported against the housing by a piston spring, so that a tensioning of the piston spring prevents any lash formation on the gas exchange valve. The force transmission to the gas exchange valve via the lash adjuster during cam loading is regulated by a control valve that controls a flow of a hydraulic medium through an axial opening between a low pressure chamber of the piston serving as an oil reservoir and a high pressure chamber that is enclosed by the piston and the housing. The control valve generally includes a closing body with a spherical or spheroid sealing surface, mostly a control valve ball that is arranged on a piston undersurface in the high pressure chamber, and a control valve spring that applies a force to the control valve ball. As explained in the following, a fundamental distinction is made in this field between two types of structures.

In a standard construction, the control valve spring is arranged as a closing operative element that presses the control valve ball with a biasing force against a valve seat on the axial opening configured as a piston bore on the piston bottom undersurface. Accordingly, the control valve of the lash adjuster is closed during a cam base circle phase when the cam of a rotating camshaft runs on the associated valve train member. During a subsequent displacement of the piston by a cam lobe, a corresponding adjusting stroke is transmitted with an adjusting force directly via the lash adjuster to the gas exchange valve that is directly operated, that is to say, opened. Because the oil in the high pressure chamber is incompressible, the lash adjuster then acts a “rigid” adjusting member. It is only upon the subsequent backward movement of the piston in the housing, when the cam re-reaches the base circle and the pressure in the high pressure chamber sinks, that the control valve opens against the force of the control valve spring and a pressure compensation takes place between the pressure chambers till the control valve spring closes the control valve again. Because in this type of construction, especially during a commencing warming-up phase of the still cold engine, a so-called pumping-up of the lash adjuster via the control valve can take place i.e., a negative valve lash leading to high engine loading accompanied by increased wear can arise, an alternative construction with an opening control valve spring has already been proposed.

Hydraulic valve lash adjusters of this type with an opening control valve spring are known as Reverse Spring Hydraulic Valve Lash Adjusters (referred to hereinafter as “RSHVA”) or Normally Open Lash Adjusters (NOLA), for example from EP 1 298 287 A2 and WO 2006 010 413 A1. In such constructions, the control valve spring is reverse-arranged, generally within the piston bore between the reservoir and the high pressure chamber, so that the control valve ball or the closing body is loaded in opening direction by a spring force and the control valve is consequently open in the cam base circle phase. In this arrangement, the control valve ball is usually received in a closing body cap that is retained on the piston bottom undersurface. The closing body cap comprises opening slots that serve as an oil passage for the control oil and a bottom for limiting the stroke of the control valve ball.

In the case of an RSHVA, a cam excursion at first causes a control oil stream to flow in closing direction from the high pressure chamber to the low pressure chamber as a result of which the lash adjuster collapses i.e., the piston and the housing are pushed together. The control oil flows around the control valve ball which, as a result, is then loaded both hydrostatically and hydrodynamically by a force against the action of the control valve spring till a resultant axial force presses the control valve ball against the valve seat and the control valve closes. The collapsing movement of the RSHVA manifests itself in a characteristic idle stroke before the actuation, properly speaking, of the gas exchange valve takes place. RSHVAs therefore act as “soft” adjusting elements that exclude a negative valve lash.

The idle stroke of the RSHVAs has an influence on the valve lift of the gas exchange valves and on the valve timing in the internal combustion engine. A corresponding speed-dependent idle stroke characteristic can be purposefully utilized in a valve or camshaft control for improving thermodynamic efficiency, for reducing pollutant emission and improving the quality of the idle stroke.

Opposed to the advantages of the RSHVA is its operational behavior that is relatively sensitive to temperature and manufacturing tolerances. In particular, due to a relatively complex geometry of the control valve components that influences the oil transfer between the pressure chambers, it is difficult to determine the hydrodynamics of such a lash adjuster. Tests have shown that a significant dependence of the closing behavior of the control valve on the oil viscosity exists practically over the entire temperature range of the engine oil from −25° to +160° C. In detail, the functioning of an RSHVA therefore depends on:

-   -   the flow geometry that is determined by the control valve         components, in particular, by the valve body geometry and the         valve seat geometry as also by flow obstacles and flow         diversions in the flow path between the pressure chambers,     -   the flushed-in contaminants (scuffing chips, impurities, foreign         bodies) in the control or engine oil,     -   the volume ratio of the high pressure chamber to the oil         reservoir,     -   the design-related so-called hydraulic rigidity of the lash         adjuster,     -   spatial displacements of the closing body and/or the control         valve spring within the receptions of these components of the         lash adjuster during operation, and     -   manufacturing and material tolerances.

Due to all these factors, comparatively large functional fluctuations can occur in this type of valve lash adjuster during operation and these manifest themselves in fluctuations of the idle stroke and the closing time as well as in an undesired temperature-dependent closing behavior and can thus detract from the actual advantages of the RSHVAs, or can at least make it more difficult to use them. An important goal in the further development of RSHVAs is therefore to minimize such interfering or difficult-to-control parameters.

The document 10 2004 018 457 A1 shows an RSHVA with different closing body geometries and guide aids for guiding the respective closing body in an associated closing body cap. According to the teaching of this document, the valve closing body can be configured, among other things, as a so-called needle formed by a ball prolonged by a circular cylindrical intermediate member. Further, the valve closing body may also comprise a spherical cup serving as a sealing surface and/or a flattened end on its cap bottom-side. The proposed guide means may be in the form of stepped recesses, cylindrical guide surfaces, central guide lugs or guide depressions as also circular cylindrical pegs on the corresponding closing body and valve cap configurations. In particular, the guide means prevents unfavorable turning out-of-place and lateral dislocations of the valve closing body. The valve closing body is surrounded with play by a cylindrical guide surface extending parallel to the axis of the RSHVA. An additional effect of this is that the actuating oil stream acts predominantly hydrostatically, so that the valve closing body is moved linearly like a piston.

The main concern of this document is to provide an RSHVA in which undesired eccentric dislocations and rotary movements caused in particular by unguided hydrodynamic flow loading can be prevented by a guided hydrostatic axial loading of the valve closing body. Accordingly, the prior art RSHVA presents an improved service performance that is less liable to fluctuate. However, this document offers no suggestions for the optimization of the other factors of influence dealt with above.

OBJECTS OF THE INVENTION

It is an object of the invention to provide an RSHVA in which the aforesaid drawbacks are largely avoided, in which, in particular, undesired functional fluctuations in operation are further reduced and which, at the same time, has a simple and low-cost structure and guarantees a reliable operation.

This and other objects and advantages of the invention will become obvious from the following detailed description.

SUMMARY OF THE INVENTION

The invention is based on the appreciation of the fact that it is possible to achieve a more stable and, in particular, a more temperature-resistant operational behavior in a hydraulic valve lash adjuster of the reverse spring type with the help of a valve closing body that reduces the free volumes in the high pressure chamber and also the difficult-to-determine hydrodynamic flow effects and possesses the simplest possible valve seat geometry.

The starting point of the invention is therefore a hydraulic valve lash adjuster for a valve train of an internal combustion engine, in particular, for an automotive vehicle, said hydraulic valve lash adjuster comprising a piston that is guided for displacement in a piston housing and elastically supported against said housing, said piston comprising a low pressure chamber that communicates via an axial opening in a piston bottom with a high pressure chamber defined by the piston housing and the piston, and further comprising a control valve acting between said pressure chambers, said control valve comprising a valve closing body that can be brought to bear sealingly against a valve seat that surrounds the axial opening on a piston body undersurface and is received in a closing body cap whose cap bottom acts as a stroke limitation of the valve closing body, and said control valve further comprising a control valve spring that loads the valve closing body in opening direction, and, during a collapsing movement between the piston and the piston housing, said valve closing body being hydraulically loadable in closing direction against the biasing force of the control valve spring by a pressure build-up in the high pressure chamber.

The invention further provides that the valve closing body is configured as a needle-shaped, elongate sealing piston whose valve seat-proximate end comprises a plate-shaped, substantially flat sealing surface that cooperates with a correspondingly configured sealing surface of the valve seat and whose valve seat-distal end comprises a surface of application for hydraulic loading, and the valve closing body substantially fills a space volume of the closing body cap surrounding the valve closing body.

The use of this construction including a high pressure chamber-reducing valve closing body advantageously results in a simpler hydraulics and a less sensitive closing behavior of the lash adjuster. A valve closing body configured as a sealing piston with a body shape comparable to a so-called valve needle but functioning with the action of a plate-like sealing element enables the use of a simple valve seat geometry of a valve plate for the control valve instead of, as usual in the past, a valve ball or a valve cone, and, at the same time, results in a significant reduction of a free dead space volume of the closing valve cap.

The reduction of the dead space of the closing body cap improves the so-called hydraulic rigidity of the lash adjuster. This is an important structural feature of a force transmission device comprising hydraulic components, particularly for the force transmission of a cam follower to a gas exchange valve, that has been left rather unheeded in the prior art reverse spring control valve constructions. Decisive for the hydraulic rigidity is the ratio of a pressure chamber volume to a pressure transmitting surface. Through the reduction of the dead space volume of the closing body cap by the needle-shaped closing body in operative cooperation with its relatively large plate-shaped sealing surface, this ratio is significantly improved in favor of the hydraulic rigidity, so that the hydraulic rigidity of the RSHVA as a whole approaches the advantageous rigidity of a purely mechanical transmission element.

Further, by the replacement of the conventional ball valve seat geometry with the particularly simple and robust plate valve seat geometry that is also less sensitive to manufacturing tolerances, the dependence of the closing behavior of the control valve on the viscosity is also advantageously reduced.

Differently from the conventional valve ball, the sealing piston is not lifted off the cap bottom of the closing body cap during the closing process due to hydrodynamic loading caused by the surrounding stream of control oil but is subjected mainly to a hydrostatic pressure that acts on a surface of application on the cap bottom-side end and produces the necessary axial closing force of the control valve. Thus, the closing body is mainly driven by the hydrostatic pressure and less by the acceleration of the oil stream during flow into the high pressure chamber.

The surface of application can be advantageously configured as a circumferential bevel on the valve seat-distal end of the valve closing body and can be hydraulically loaded through an oil passage in the closing body cap. The sealing piston can be economically arranged in a closing body cap of conventional shape fixed to the piston bottom.

As a result, due to the flat sealing geometry of the valve seat and the predominantly hydrostatic activation of the closing body that substantially fills the closing body cap, a more stable closing and idle stroke behavior of the RSHVA is obtained with respect to temperature and oil viscosity variations as well as manufacturing tolerances.

Further, according to another proposition of the invention, the valve closing body may be guided with guide clearance in the closing body cap. Because, according to the invention the sealing piston anyhow fills the cap substantially, so that its periphery preferably extends adjacent to the inner wall of the closing body cap, guidance with guide clearance advantageously enables automatic centering and guarantees a high tilting stability of the closing body.

In addition to this simplest guiding means, it is obviously possible to arrange on the valve closing body and/or the closing body cap, a more sophisticated guiding means with a higher guiding precision and broadened guiding functions that acts as an anti-tilt device and/or as a centering means and/or as an anti-rotation device of the valve closing body. In particular, an additional anti-rotation device of the closing body can be of advantage for a reliable exclusion of undesired rotary movements of the closing body in operation.

The tilting stability can also be advantageously augmented if at least a central region of the valve seat-distal, cap bottom-side end of the valve closing body has a shape mating that of the cap bottom. In this way, no unnecessary dead space is left free. In addition, in the open state of the valve, the control valve spring brings the closing body to bear flatly against the cap bottom that serves as a stroke limitation of the control valve. As a result, a uniform opening gap is formed on the valve seat that favors a perfect opening and closing function as well as a uniform oil transfer of the control valve.

According to another proposition of the invention, a dirt-collecting device may be configured in the valve closing body. Extremely fine pollutants like scuffing chips, manufacturing residues and impure oil that can lead to operational fluctuations or malfunctioning, or intensify these, can be transported into the RSHVA through the oil supply of the oil reservoir (low pressure chamber). With the help of a dirt-collecting device, such undesired effects can be largely excluded.

An advantageous dirt-collecting device can be configured in the valve closing body as a recess that is arranged coaxially to the axial opening and is open on the valve seat-proximate end of the valve closing body. A particularly simple and effective embodiment is a relatively deep cylindrical collecting pocket in the form of a bore made in the closing body to receive flushed-in particles. These particles can be particularly reliably collected in a bottom-side sink of the recess, so that it is hardly possible for them to re-enter the oil circulation. As a result, more sophisticated filters or particle separators arranged upstream in the low pressure chamber can thus be dispensed with as a rule. In addition to its dirt-collecting function, the recess also contributes to a reduction of weight of the valve closing body.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described more closely in the following with reference to one example of embodiment illustrated in the appended drawings.

FIG. 1 shows a longitudinal section of a portion of an RSHVA comprising a control valve of the invention shown in open position, and

FIG. 2 shows a longitudinal section of a valve closing body of the RSHVA of FIG. 1 on an enlarged scale.

DETAILED DESCRIPTION OF THE DRAWINGS

The RSHVA shown in FIG. 1 is advantageously configured as a hydraulic tappet 1, for example a cup tappet or a roller tappet of a valve train of an internal combustion engine in an automotive vehicle.

The tappet 1 comprises a cylindrical piston 2 that is guided for axial displacement with a sealing clearance in a piston housing 4. A cylindrical recess in the interior of the piston 2 forms a low pressure chamber or oil reservoir 3 that is supplied with control oil through a feed arrangement, not illustrated. A central piston bore 26 arranged in a front end piston bottom 19 of the piston 2 connects the low pressure chamber 3 to a high pressure chamber 5 that is defined by the piston bottom 19 and the housing 4. The axial opening 26 can be loaded by a control valve 6 that controls a control oil exchange between the two pressure chambers 3 and 5.

The control valve 6 comprises a valve closing body 7 configured according to the invention, a control valve spring 8 configured as a coiled compression spring and a closing body cap 9. The control valve spring 8 is supported in direction of the low pressure chamber 3 on a support surface 27 of the piston bore 26 and biases the valve closing body 7 with a spring force in opening direction, i.e. towards the high pressure chamber 5.

For more clarity, FIG. 2 shows an enlarged representation of the valve closing body 7. This is configured as a cylindrical sealing piston that comprises a plate-like sealing surface 10 on its valve seat-side end. On its valve-distal end, the closing body 7 has a flat configuration in a central region 11. The central region 11 is surrounded by a circumferential chamfer that forms a surface of application 12 for the control oil. The invention further provides a dirt-collecting device 13 that is advantageously configured as a cylindrical, valve seat-side central bore in the valve closing body 7 with a bottom-side sink 14 and collects and stores any pollutants flushed in through the oil reservoir 3.

The closing body cap 9 comprises a collar-shaped cap flange 15 with which it is advantageously retained by a clamp joint on the undersurface 28 of the piston bottom 19 in a circular recess 16 that tapers slightly towards the front end of the piston bottom 19. A piston spring 20 configured as a coiled compression spring is supported between the cap flange 15 of the closing body cap 9 and a housing bottom 18 of the housing 4.

The valve closing body 7 is guided with a radial guide clearance in the closing body cap 9 that is filled by the closing body 7 with the exception of a region 17 remaining free due to the chamfer 12 for oil reception and a small volume left free in the region of the cap flange 15. A free space volume of the high pressure chamber 5 is accordingly reduced by the space volume occupied by the closing body 7. Through a cap oil passage 23, for example, in the form of one or more slot-like openings in the side wall region of the closing body cap 9, the oil-receiving region 17 can be fed with control oil from the outer part of the high pressure chamber 5, mainly for the hydrostatic loading of the surface of application 12.

In the open state of the valve shown in FIG. 1, the valve closing body 7 is pressed by the biased control valve spring 8 against a cap bottom 21 of the closing body cap 9, so that the central region 11 of the valve closing body 7 bears flatly with mating contact against the cap bottom 21. The cap bottom 21 acts a stroke limitation for the closing body 7 in the opening direction of the control valve 6. In this way, the width of an opening gap 22 and thus a closing body stroke of the control valve 6 is defined. This position corresponds to a camshaft position in the cam base circle phase.

Upon a loading through a cam lobe of a cam of the rotating camshaft of the internal combustion engine, the piston 2 and the housing 4 are pushed together, so that the RSHVA tappet 1 at first produces an idle stroke during which oil flows through the opening gap 22 and the piston bore 26 into the low pressure chamber 3. At the same time, an increasing hydraulic pressure is built up in the high pressure chamber 5, and this pressure loads the valve closing body 7, more precisely, its surface of application 12, substantially hydrostatically and brings the closing body 7 to bear, against the action of the control valve spring 8, against a flat sealing surface 25 of a valve seat 24 corresponding to the plate-like sealing surface 10 of the closing body 7, so that the control valve 6 closes. Subsequently, further mechanical loading through the cam is transmitted through the now incompressible high pressure chamber 5 to the gas exchange valve to be actuated, and this opens.

With the further rotation of the camshaft, the cam returns relative to the tappet 1 into the cam base circle so that the gas exchange valve closes. The piston spring 20 presses the piston 2 and the housing 4 so far apart that no more lash exists between the tappet 1 and the camshaft, more precisely, the cam. During this time, the pressure in the high pressure chamber 5 falls and the control valve 6 opens, so that control oil can flow into the high pressure chamber 5. 

1. A hydraulic valve lash adjuster for a valve train of an internal combustion engine, said hydraulic valve lash adjuster comprising a piston that is guided for displacement in a piston housing and elastically supported against said housing, said piston comprising a low pressure chamber that communicates via an axial opening in a piston bottom with a high pressure chamber defined by the piston housing and the piston, and further comprising a control valve acting between said pressure chambers, said control valve comprising a valve closing body that can be brought to bear sealingly against a valve seat that surrounds the axial opening on a piston body undersurface and is received in a closing body cap whose cap bottom acts as a stroke limitation of the valve closing body, and said control valve further comprising a control valve spring that loads the valve closing body in opening direction, and, during a collapsing movement between the piston and the piston housing, said valve closing body being hydraulically loadable in closing direction against the biasing force of the control valve spring by a pressure build-up in the high pressure chamber, wherein the valve closing body is configured as a needle-shaped, elongate sealing piston whose valve seat-proximate end comprises a plate-shaped sealing surface that cooperates with a correspondingly configured sealing surface of the valve seat and whose valve seat-distal end comprises a surface of application for hydraulic loading, and the valve closing body substantially fills a space volume of the closing body cap surrounding the valve closing body.
 2. A hydraulic valve lash adjuster of claim 1, wherein the valve closing body is guided in the closing body cap with a guide clearance.
 3. A hydraulic valve lash adjuster of claim 1, wherein a guide means is arranged or configured on the valve closing body and/or the closing body cap.
 4. A hydraulic valve lash adjuster of claim 3, wherein the guide means acts as a anti-tilt device and/or a centering means and/or an anti-rotation device of the valve closing body.
 5. A hydraulic valve lash adjuster of claim 1, wherein at least a central region of the valve seat-distal, cap bottom-side end of the valve closing body has a shape mating the cap bottom.
 6. A hydraulic valve lash adjuster of claim 1, wherein the surface of application is configured as a circumferential chamfer on the valve seat-distal end of the valve closing body and can be hydraulically loaded through an oil passage in the valve body cap.
 7. A hydraulic valve lash adjuster of claim 1, wherein a dirt-collecting device is configured in the valve closing body.
 8. A hydraulic valve lash adjuster of claim 7, wherein the dirt-collecting device is configured in the valve closing body as a recess that is arranged coaxially to the axial opening and is open on the valve seat-proximate end of the valve closing body.
 9. A hydraulic valve lash adjuster of claim 8, wherein the recess is configured as a cylindrical collecting pocket.
 10. A hydraulic valve lash adjuster of claim 8, wherein the recess comprises a bottom-side sink. 